This invention relates to electrostatic copying machines using a copy sheet having an electrically conductive layer, such as for instance, to electrostatic transfer type copying machines, in which the electrostatic latent image that has been formed on a photoconductive drum is transferred onto a copy sheet having a dielectric layer overlaying its electrically conductive layer. The electrostatic latent image thus obtained is developed and fixed. The invention also relates to electrostatic copying machines in which the electrostatic latent image is directly formed on a copy sheet having a photoconductive layer overlaying its electrically conductive layer. The electrostatic latent image thus obtained is developed and fixed.
At the present time, there has arisen a demand for reducing the size of copying machines and improving the clearness of the images obtained. Also there is a need for speeding-up the copying operation, particularly the copying operation for a single copy sheet. To meet such demands, there have been proposed many arrangements which are intended to improve the image-forming elements such as the electrostatic-latent-image-forming devices, the developing devices, the fixing devices and so forth. Also the copy-sheet-transporting passages between the respective image-forming elements have been shortened. Particularly, it is essential to provide a transporting passage with a length shorter than that of the copy sheet, for achieving a reduced size of the copying machine and for speeding-up the copying operation.
Further, to achieve the desired improved clearness of the image, it is essential that the device for developing the electrostatic latent image on the copy sheet be of the developing-electrode type.
A fixing device using a heated drum contributes to a reduction in the size of the copying machine. In this connection, for achieving a rapid copying operation by using a heated drum, it is essential that the copy sheet be brought into intimate contact with the surface of the heated drum for ensuring a rapid fixing. That object is attained by uniformly charging the surface of the heated drum by a corona charger, so that the copy sheet is electrostatically attracted to the surface of the heated drum.
Moreover, in the case of a dry type electrostatic copying machine, after development the copy sheet retains an unfixed toner image, so that the means for transportation of the copy sheet to a fixing station and through the fixing device should be of a type, in which only one side of the copy sheet is utilized for transportation. For example, in electrically conductive layer of the copy sheet is utilized for transportation, unlike the transportation of the copy sheet up to the developing station where both sides of the copy sheet may be used in its conveyance. Thus, it has been general practice to use an endless belt transportation system, and the surface of a heated drum is commonly used as a transporting means through the fixing station. Accordingly, for ensuring the positive transportation of a copy sheet during rapid copying operation, it is mandatory that the copy sheet be positively attracted to the transporting means. To reduce the size of a copying machine, while achieving positive attraction and transportation of copy sheets, it is preferable to adopt an electrostatic attracting system using, for example, a corona discharge device.
Also, in the case of a dry type electrostatic copying machine, the improvements in the clearness of an image dictate the use of a developing-electrode type developing device, which uses a mono-component toner having magnetic properties and electrical conductivity, as is disclosed in U.S. Pat. No. 3,909,258, so that electrically conductive paths in the form of toner chains are formed between the electrostatic latent image surface of a copy sheet and the developing electrode.
Although the electrostatic copying machine of the type described is successful in reducing the size of a copying machine and in obtaining rapid copying operation according to respective improvements in the image-forming-elements and the lengths of the transporting passages, the machine still poses another unexpected problem with respect to the clearness of an image.
The problem observed by the inventor in electrostatic copying machines embodying in developing device of the conventional developing electrode type, is that, with the leading portion of the copy sheet having passed the developing station and reached the transporting belt to be fed in electrostatically attracted condition and the remainder of the copy sheet still within the developing station, upon passing through the developing station a uniform ground contamination is formed thereon by the adhesion of toner onto the area having no electrostatic latent image. Ground contamination also appears on the copy sheet after the leading portion thereof has reached the heated drum in an electrostatically attracted condition which accelerates the adhesion of toner to the copy sheet.
Furthermore, the inventor has found that in case the charging polarity of the corona charger is opposite to that of an electrostatic latent image on a copy sheet, the contrast of the developed image is further impaired. This is caused because, upon development of an electrostatic latent image on a copy sheet with the aid of electrically conductive paths in the form of toner chains between a developing electrode and the aforesaid latent image, the potentials which have been charged on the surface of the transporting belt as well as on the heated drum are impressed on the electrically conductive layer of the copy sheet, so that there is a potential difference between the developing electrodes and the copy sheet through the medium of the electrically conductive layer thereof. Thereby, electrically conductive paths in the form of toner chains are formed even in the non-image areas of the copy sheet, leading to the clinging of toner to the copy sheet.
Stain or contamination due to toner on the copy sheet will be described in more detail in conjunction with the description of FIG. 1. Copy sheet 10 which has dielectric layer 6 deposited on electrically conductive layer 7 receives thereon a negative electrostatic latent image. Developing electrodes 8a, 8b are grounded to the body proper of the copying machine (not shown). Voltage impressing means 9 for impressing a voltage on electrically conductive layer 7 of the copy sheet, consists of a corona charger for charging the surfaces of the transporting belt and the heated drum, and is grounded to the body proper of the copying machine. The description is made with a positive voltage impressed on electrically conductive layer 7 of copy sheet 10, and copy sheet 10 is moved in the direction of the arrow.
As will be apparent from the drawing, developing electrode 8a and voltage impressing means 9 are disposed so that an electrically conductive path is formed through the body proper of the copying machine and electrically conductive layer 7 of copy sheet 10. The potential of electrically conductive layer 7 of copy sheet 10 positioned between both developing electrodes 8a, 8b is raised to a positive electric potential as shown, the absolute value of which is higher than that of the reference potential. The positive electric potential thus raised produces a potential difference, i.e., an electric field, between electrically conductive layer 7 and developing electrode 8a. That electric field exerts a biasing action, i.e., an inverse biasing action in the example shown, on the developing electric field produced by the electrostatic latent image on the copy sheet. The electric field provides an electrically conductive path in the form of toner chains even in the area of the copy sheet from which an electrostatic latent image is absent. The electric field acts to weaken the developing electric field on the electrostatic-latent-image-formed-area of the copy sheet, the aforesaid developing electric field forming an electrically conductive path in the form of toner chains, thus adversely affecting the clearness of the reproduced image.
To solve the aforesaid problem, a grounding method has been proposed, in which, as is exemplified in FIG. 2, there is provided between developing electrodes 8a, 8b and voltage impressing means 9 a means 11 for removing or dissipating the electric potential from electrically conductive layer 7 of copy sheet 10. In other words, electrically conductive members grounded to the body proper of the copying machine are brought into contact with electrically conductive layer 7 of copy sheet 10, so as to lower the electric potential on electrically conductive layer 7 to the proximity of a reference electric potential. Such a grounding system, however, is not effective to lower the electric potential on electrically conductive layer 7 of copy sheet 10 to the proximity of the reference electric potential, because electrically conductive layer 7 has an electric resistance in the order of 10.sup.6 - 10.sup.9 ohm, and also because there is an unstable contact between the copy sheet and the electrically conductive members. To lower the electric potential to the proximity of the reference potential successfully, an inverse biasing system has been proposed, in which electrically conductive members are sequentially disposed in contacting relation to electrically conductive layer 7 of copy sheet 10 between voltage impressing means 9 and developing electrodes 8a and 8b. In that manner one electrically conductive member impresses on electrically conductive layer 7 a bias voltage of a polarity opposite to that of voltage impressing means 9. The other electrically conductive member in succession thereto impresses on electrically conductive layer 7 a bias voltage having gradually reduced absolute values with a polarity opposite to that of the first electrically conductive member, so that the electric potential on electrically conductive layer 7 is lowered into close proximity to the reference potential immediately in front of developing electrodes 8a and 8b. Such an inverse biasing system is disclosed in U.S. Pat. No. 3,757,165.
The techniques as are taught in U.S. Pat. No. 3,757,165 are effective only for electrically conductive members, such as copy-sheet transporting rollers which are in complete intimate contact with the electrically conductive layer of the copy sheet.
Accordingly, for a copy sheet which retains unfixed toner after development, such transporting rollers for transporting the copy sheet prior to its development are not recommended in view of the tendency of toner to cling to the rollers. Additionally, the aforesaid one-side transportation utilizing the electrically conductive layer of the copy sheet does not ensure intimate contact between the copy sheet and the roller. For the above reasons, the above-described means for removing electric potential from the electrically conductive layer are not adapted for use in a copying machine.
The stain on the copy sheet is a problem to be solved in satisfying demands for reducing the copying machine size, improving clearness of the reproduced image and accelerating the copying operation.